Transducer having mechanical impedance matching between air and the driver



Qui -34w Oct. 20, 1970 H. w. 'SCHAFFT 3,535,471 TRANSDUCER HAVING MECHANICAL IMPEDANCE MATCHING BETWEEN AIR AND THE DRIVER Original Filed June 17, 1965 FIG. 1

INVENTOR Hugo W Schaffr -United States Patent C) 3,535,471 TRANSDUCER HAVING MECHANICAL IMPED- ANCE MATCHING BETWEEN AIR AND THE DRIVER Hugo W. Scliatrt, Des Plaines, Ill., assignor to Motorola, Inc., Franklin Park, "L, a corporation of Illinois Continuation of application Ser. No. 464,769, June 17, 1965, which is a continuation-in-part of application Ser. No. 423,683, Jan. 6, 1965. This application Sept. 10, 1969, Ser. No. 856,850

Int. Cl. H04r 17/00 US. Cl. 179-110 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to sound signal transducers and in particular to a loudspeaker using a signal responsive dimensionally changeable member as a driving element. -The present application is a continuation of my application Ser. No. 464,769, which is a continuation-in-part of my application Ser. No. 423,683, filed Jan. 6, 1965, now abandoned.

Dynamic loudspeakers used to produce high frequency sound signals from electrical signals applied thereto may have various problems associated with their operation. The dynamic speaker normally operates above its resonant frequency and therefore requires considerable power to drive the speaker diaphragm. When it is desired to shape the frequency response curves .of one dynamic speaker to be used with another, electrical cross-over networks are required further complicating the speaker system. When operating at a frequency above the resonant frequency of the driver, transients may become a problem, particularly as compared to operating the speaker below resonance. When signals above and below the resonant frequency of the speaker system are reproduced there will not be a smooth transition as the reproduced signals pass through the resonant frequency and phase errors will be introduced. Other prior speakers with solid driver elements have not provided fully satisfactory efiiciency due in large part to poor coupling or matching of the transducer to the air. Previously developed speakers, either dynamic or of some other type, have suffered from these difficulties and the cost of manufacture of such speakers or associated circuits has been increased in the attempts to overcome the defects.

It is, therefore, an object of this invention to overcome one or more of these problems and to provide an improved and simplified speaker capable of manufacture at reduced cost.

Another object is to increase the efiiciency of a high frequency loudspeaker or horn.

'Another object of this invention is to provide a speaker operative within a band of frequencies and usable with I another speaker without the requirement. of electrical cross-over networks.

A feature of this invention is the provision of a'speaker 3,535,471 Patented Oct. 20, 1970 ice erating as a length expander and directly coupled to the speaker diaphragm.

Another feature of this invention is the provision of a speaker having an elongated dimensionally changeable driving element and in which the speaker reproduces signals below the longitudinal resonant frequency of the driving element and the cross-sectional area of the driving element is selected to provide improved coupling of the speaker to air.

Another feature of this invention is the provision of a speaker having a tapered ceramic driving element to increase the cutoff frequency of the speaker for a given length ceramic element.

Another feature of this invention is. the provision of a speaker having a driving element consisting of a plurality of ceramic elements connected in a multielement length expander configuration.

The invention is illutrated in the drawings wherein:

FIG. 1 is a cross-sectional view of a speaker using a ceramic driving element;

FIG. 2 is a cross-sectional view of the ceramic driving element of FIG. 1;

FIG. 3 is a drawing of a flat plate ceramic driving element;

FIG. 4 is a drawing of a flat plate ceramic driving element having a linear taper;

FIG..5 is a drawing of a fiat plate ceramic driving element having an exponential taper; and

FIG. 6 is a drawing of a multielement ceramic driving element operating as a length expander.

In practicing this invention, a horn type speaker is provided for producing high frequency sound signals in response to electrical signals applied thereto. The speaker includes a frame and a diaphragm mechanically supported by the frame. An elongated driving element, such as a ceramic member is connected between the frame and the diaphragm which operates as a piston. Electrodes are positioned on the ceramic member and the electrical signals are applied to the electrodes to cause the ceramic element to vibrate as a one-quarter wave length expander operating below its resonant frequency. The changes in length of the vibrating element cause the diaphragm to move and develop sound signals. The diameter of the diaphragm is made as large as feasible and the air loading in the compression chamber of the horn is made relatively great to provide increased mechanical impedance for the diaphragm to present to the driving element. Selection of the cross-sectional area of the driving element then permits a desired approach to matching of mechanical impedances between the air and the driving member. As explained subsequently in greater detail the shape of the frequency response curve of the speaker device is determined by this impedance matching between the length expander and the air to which it is coupled. By using a length expander as described, the resonant frequency thereof can be established independently of its cross-sectional area so that very satisfactory control of the efficiency and overall frequency response of the speaker device is possible.

The ceramic type driving element can take many shapes including a hollow cylindrical tube or a thin fiat plate. The flat plate can be rectangular in shape or can have tapered sides to increase the propagation velocity and thus the maximum cutoff frequency for a given length of ceramic crystal. Several ceramic members can be combined in a multielement length expander configuration to increase the power available for driving the diaphragm. Damping members can be added between the crystal and the frame to reduce the tendency of the crystal to vibrate transverse to its length.

having a dimensionallychangeable driving element op- A horn type speaker incorporating a ceramic crystal driving element is illustrated in FIG. 1. The speaker consists of a round rear frame portion 12 and a round front frame portion 23 including flared horn 21. A baffle or compression chamber plate and a diaphragm 13 are positioned in spaced relation between front frame portion 23 and the rear frame portion 12. Compression chamber plate 15 clamps diaphragm 13 to rear frame portion 12. Vibrations of diaphragm 13, acting as a piston, force air from the space between diaphragm 13 and plate 15 through openings 22 in compression chamber plate 15 thus producing sound signals.

Rear frame portion 12 includes a driver support plate 24. Positioned between driver support 24 and diaphragm 13 is a ceramic crystal 14 in the form of a hollow cylindrical tube. Ceramic crystal element 14 is cemented to driver support 24 at 17 and to diaphragm 13 at 19.

&\ damping support positioned between driver support 12 and ceramic element 14, supports the crystal between end points 17 and 19 to reduce the tendency of element 14 to vibrate transversely to its length. Wires 16 and 18 are connected to terminal strip 25 and to electrodes 26 and 27 on the outside and inside surfaces of element 14 (FIG. 2) to apply signals to the crystal causing it to vibrate as a length expander. Note that the field between electrodes 26 and 27 will be in a direction such that correct polarization of the ceramic tube gives the length vibration. While the ceramic element 14 is shown driving a diaphragm used in a compression type horn speaker it can also be used to drive some other type of speaker diaphragm as, for example, a paper cone.

Ceramic crystal element 14 is operated at a frequency below its resonant frequency and thus its length is made equal to one-quarter wave length at a frequency as high as the highest frequency it is desired reproduced. A frequency above this is called the cutoff frequency as the output of the ceramic element 14 falls off rapidly when frequencies higher than the resonant frequency are applied to the crystal (due to the capacitance of .the element). The impedance of the ceramic member gradually increases with decreasing frequency to establish a lower frequency limit to the speaker. Thus, a speaker using a ceramic driver has a natural band of frequencies in which it operates without requiring electrical crossover networks.

The effective length or crystal element 14 is reduced by the mass of the diaphragm cemented to one end, and also by the cement used to connect the crystal to diaphragm. 13 and driver support 22. Thus the effective length of element 14, which is used to establish the cutoff frequency of the speaker, is slightly less than the physical length of the ceramic crystal itself. In an example of a crystal in which the cutoff frequency was to be 12.5 kc., the nominal quarter wavelength, for this frequency, of 5.71 cm., was decreased to 5.60 cm. to allow for the mass of the cement and the diaphragm. Thus, the effective length of the 5.71 cm. crystal in the above example would be 5.60 cm. Making the mass of the ceramic tube 14 high compared to the mass of the diaphragm, reduces the effect of the diaphragm.

The diameter of diaphragm 13 is made relatively large (as compared with those generally used with other horns) and in the specific case under consideration the element is spun aluminum with an effective piston diameter of approximately 1% inches. Diaphragm 13 is also shaped elliptically so that its stiffness is increased and it will not flex in an undesired manner as it is operated as a piston by longitudinal motion of the rod 14. It is also possible to reduce the size of the compression chamber between diaphragm 13 and baffie plate 15 by reducing the spacing between these numbers. In the present example the spacing is .013 inch. This, along with a large dia-meter for the diaphragm 13 will tend to raise the mechanical impedance to improve the coupling from the mechanical impedance of this driver element can be controlled, independently of control of its resonant frequency determined primarily by its length, by varying its crosssectional area. Element 14 is shown tubular in order to provide desirable mechanical rigidity while at the same time permitting a relatively small cross-sectional area to lower its mechanical impedance and come closer to matching that of the air loaded diaphragm.

Since the rod element 14 is resonant, it will, of course, have a pronounced response at its resonant frequency. However, it is usually desirable that the width of the usable frequency response be controllable and that the slope, or rate of change of that response with frequency, be controllable so that the horn can be matched with other loudspeakers with which it may be used or with some other criteria, such as the response of the ear to different frequencies. This slope of the frequency response curve and the maximum amplitude of it can be regulated in accordance with the load placed on the driver element so that in effect its loaded Q is very much lower than its unloaded Q in free vibration. Thus, the amount of the highest amplitude of response as well as the shape of the slope of the response approaching that maximum frequency response is a function of the amount of the loading on the driver element 14. It may be seen that the response curve will be effectively reduced in amplitude and broadened by reducing the cross-sectional area of the driver element 14 or by increasing the area of the diaphragm 13 or by reducing thecompression chamber spacing between diaphragm 13 and baffle plate 15.

It is also possible to construct driver element 14 of magnetostrictive material. Such a driver would be driven by a magnetic coil so that its length varied in accordance with the audio signals. A permanent magnet longitudinally aligned with and coupled to the rod 14 would provide a magnetic bias to avoid frequency doubling.

FIG. 3 illustrates another form of this invention in which crystal element 30 is in the form of a thin flat plate, shown edge on in this view. Electrodes 31 and 32 are positioned on opposite sides of the plate and are connected to the source of the electrical signal by wires 33 and 34. The ends 38 and 39 of ceramic crystal 30 are cemented to driver support 24 and diaphragm 13 respectively as were the ends of the hollow cylindrical tube crystal of FIGS. 1 and 2. The electrical signal applied to electrodes 31 and 32 develop a field across crystal 30 which causes the crystal to vibrate as a length expander, thus causing diaphragm 13 to vibrate.

FIG. 4 illustrates another embodiment in which the ceramic crystal 40 is in the form of a thin flat plate having its sides tapered towards the end of the crystal cemented to diaphragm 13, so that the area of the plate decreases from driver support 24 to diaphragm 13. The ends of ceramic crystal 40 are cemented to driver support 24 at point 43 and diaphragm 13 at point 42. An electrode 41 is positioned on one surface of the crystal 40. Another electrode, not shown, is positioned on opposite surface. Wires 45 and 46 are coupled to the electrodes for applying the electrical signal thereto. The electrical signal applied to the electrodes develops an electric field across ceramic crystal 40 causing it to vibrate as a length expander and thus causing diaphragm 13 to vibrate.

, FIG. 5 shows an embodiment in which crystal is given an exponential taper so that the area of the plate decreases exponentially. The ends of crystal 60 are cemented to driver support 24 at point 63 and diaphragm 13 at point 62. An electrode 61 is positioned on one surface of crystal 40 and another electrode, not shown, is positioned on the opposite surface. Wires 65 and 66 are coupled to the electrodes for applying the electrical signals thereto. The electric signal applied to the electrodes develops an electric field across crystal 60 causing it to vibrate as a length expander and thus causing diaphragm 13 to vibrate.

Tapering the sides of ceramic crystals 40 and 60 in this manner increases the velocity of propagation of the compressional wave in the crystal and thus results in a higher resonant frequency for a given length of crystal. This form of construction permits the extension of the cutoff frequency of the system without decreasing the length of the ceramic crystal. The tapers shown in FIGS. 4 and 5 are linear and exponential tapers. However, the taper may be gaussian, stepped or any other function which will give desired results.

FIG. 6 shows an embodiment of the invention in which a plurality of crystals 50, spaced apart by electrodes 51 and 52 are fastened together in a multielement length expander configuration. Electrodes 53 and 54 are positioned on the outside surfaces of the plurality 'of crystals. The electrical signals are applied to electrodes 51, 52, 53 and 54 through wires 55 and 56. The crystals 50 are polarized and the electrodes are energized so that the length of each crystal changes 'in the same direction as the adjacent crystals when the electrical signals are applied to the crystals. This form of construction permits the diaphragm 13 to be driven with a greater force than would be possible with a single crystal. Thus, a simply constructed speaker system using a dimensionally changeable driver has been shown. The driver operates as a length expander and is coupled directly to the speaker diaphragm. The speaker reproduces a band of frequencies with a minimum of transient distortion and without requiring electrical crossover networks, while at the same time permitting a favorable manufacturing cost.

I claim:

1. A horn type speaker for producing sound signals in response to audio frequency electrical signals applied thereto, including in combination:

- a frame;

a diaphragm mechanically supported by the frame and having an elliptical cross-sectionalv configuration along the axis thereof;

" a perforated bafile plate spaced a predetermined distance from the diaphragm;

a flared sound director positioned to receive and direct sound passing through the baffle plate;

means including an elongated tubular member of thin cross-section capable of length change in response to signals applied thereto, the member having a pair of opposing ends, one of the ends being mechanically secured to the frame and the other of the ends being secured to the diaphragm, the member having a length selected to provide longitudinal resonant 7 action thereof at a frequency greater than the highest frequency to be reproduced by the speaker, and means for applying the electrical signals to the member so that the same vibrates as a length expander, with the area and shape of the diaphragm, the spacing between the diaphragm and the baffle plate, and the cross-sectional area of the elongated member all being selected to provide a predetermined mechanical impedance match between the elongated member and the air in the flared horn director.

2. A speaker for producing sound signals in response to audio frequency electrical signals applied thereto including in combination;

a frame;

a diaphragm mechanically supported by the frame, having a non-linear cross-sectional configuration along the axis thereof and having a driving portion;

a 'bafiie plate spaced a predetermined distance from the diaphragm;

a sound director positioned to receive and direct sound passing through the baflle plate;

an elongated tubular member of a piezoelectric ceramic material having a thin cross-section and capable of vibrating as a length expander in response to signals applied thereto, the member having a pair of opposing ends, one of the ends being mechanically secured to the frame and the other end being secured to the driving point of the diaphragm, with the crosssectional area of the tubular member, the spacing between the diaphragm andthe baflle plate, and the area and cross-sectional configuration of the diaphragm all being selected to provide a predetermined mechanical impedance match between the tubular member and the air loading the diaphragm; and circuit means connected to the tubular member for applying electrical signals to the member, the length of the tubular member being selected to become longitudinally resonant at a frequency near the upper frequency range to be reproduced by the speaker. 3. The combination according to claim 2 wherein the resonant frequency of the tubular member is at a frequency greater than the highest frequency to be repro- H References Cited UNITED STATES PATENTS 1,688,744 10/1928 Nicolson 179-110 1,802,781 4/1931 Sawyer 179110 2,479,987 8/1949 Williams 179 110 3,068,446 12/1962 Ehrlich 179-1l0 3,215,977 11/1965 Williams 310-8.7 3,310,131 3/1967 Ward 1s1 32 OTHER REFERENCES Beranek, L. L.: Acoustics, N.Y., McGrawHill, 1954,

RALPH D. BLAKESLEE, Primary Examiner US. Cl. X.R. 310-8.7 

